Since the early 1960's, quartz crystals have been used to monitor thin film coating processes used in the fabrication of optical devices such as lenses, filters, reflectors and beam splitters. Although initially employed as an aid to optical monitors to provide information on the rate at which the film is deposited, quartz crystal sensors became relied upon to indicate and control optical layer thickness in automated deposition systems.
Research in fields such as nanotechnology, biosensors, thin film displays, and high-speed optical communications have increased the complexity of thin film structures. While an antireflection coating consisting of a single layer of magnesium fluoride may have been sufficient 20 years ago, current designs may call for a 24-layer stack of alternating refractive index films. With high-speed optical communications, this stack increases ten-fold, leading to filters comprised of up to 256 layers.
The manufacturing of these geometries requires the control and accuracy provided by a quartz crystal. Unfortunately, the materials and deposition temperatures utilized in today's processing can adversely affect the operation of the crystal sensor.
Quartz crystal thickness monitors may be the most misunderstood components of optical thin film deposition systems. Quartz sensors provide process engineers with coating rate and thickness data in real time, with Angstrom resolution.
Quartz sensor instruments measure film thickness by monitoring a change in the frequency of vibration of a test crystal coated simultaneously with process substrates. Quartz is a piezoelectric material., i.e., if a bar of quartz is bent, it will develop a voltage on opposite faces. Conversely, if a voltage is applied, the bar will bend. By applying alternating voltage to such a bar, the bar will vibrate or oscillate in phase with the voltage.
At a specific frequency of oscillation, quartz will vibrate with minimal resistance, much like a tuning fork rings when struck. This natural resonance frequency is used as the basis for measuring film thickness. By adding coatings to the crystal surface, the resonance frequency decreases linearly. If the coatings are removed, the resonance frequency increases.
In a quartz crystal thickness monitor, the quartz crystal is coupled to an electrical circuit that causes the crystal to vibrate at its natural (or resonant) frequency, which for most commercial instruments is between 5 and 6 MHz. A microprocessor-based control unit monitors and displays this frequency, or derived quantities, continuously. As material coats the crystal during deposition, the resonant frequency decreases in a predictable fashion, proportional to the rate material arrives at the crystal, and the material density. The frequency change is calculated several times per second, converted in the microprocessor to Angstroms per second and displayed as deposition rate. The accumulated coating is displayed as total thickness.
The sensitivities of these sensors are remarkable. A uniform coating of as little as 10 Angstroms of aluminum will typically cause a frequency change of 20 Hz, easily measured by today's electronics. As the density of the film increases, the frequency shift per Angstrom increases.
The useful life of quartz is dependent on the thickness and type of coating monitored. If a low stress metal such as aluminum is deposited, layers as thick as 1,000,000 Angstroms have been measured. At the other extreme, highly stressful dielectric films can cause crystal malfunction at thicknesses as low as 2,000 Angstroms or less.
In the early days of crystal thickness monitors, metallic films of copper, silver and gold were the materials that were deposited the most commonly. These films produced coatings of low stress and were condensed on substrates held near room temperature. Under these conditions, very accurate determinations of film thickness and rate were achievable.
When the optics industry began to employ crystal monitors, attention shifted from opaque metals to transparent materials such magnesium fluoride, and silicon dioxide, since coatings had to transmit light. Unfortunately, these substances produced films with high intrinsic stresses and required high process or substrate temperatures. These were not welcome developments for crystal monitoring, as sensors which have employed quartz have been highly sensitive to stress and temperature changes.
This sensitivity can be traced to the piezoelectric properties of quartz. Further complicating matters is the fact that quartz crystal sensors which have been employed have exhibited frequency change when deformed by thin film stresses or mechanical forces, e.g., from a mounting holder. If process conditions heat or cool such sensors, a similar frequency shift occurs. Regardless of the origin, the frequency shift is indistinguishable from that caused by the addition of coating.
Frequency shifts can be positive or negative, and can be cumulative. They can also be random. Causes of resonant frequency changes include:
Vibrations introduced through the mounting hardware;
Variations in the voltage used to oscillate the crystal;
Changes in the film being monitored (acoustic impedance);
Adhesion failure of the monitored coating or quartz electrodes; and
Radio frequency interference in the monitoring circuit.
These effects introduce large errors in thickness and rate calculations. Temperature swings in quartz can result in thickness variations of 50 Angstroms or more (see FIG. 1, which is a plot of frequency shift vs. temperature for AT-cut quartz crystal). Adhesion failure results in 100-Angstrom rate spikes. Extraneous vibrations can produce changes in the thousand Angstrom range. For precision optical components, these errors result in major yield loss.
The harsh conditions present during optical film coating can have deleterious effects on the operating life of a crystal. High stress coatings can deform the crystal to the point that it ceases to oscillate, without warning. Splatters of material from the coating source can lead to similar failure. High-energy plasmas used for substrate cleaning can couple into the crystal electronics and cause severe electrical noise. High temperature depositions can overheat the crystal, driving it past its operating limit.
Early crystal failure can be a great inconvenience or an unmitigated disaster. In the case of 100+ layer thin film stacks, venting the chamber to replace crystals is not an option, due to the undesirable effects of atmospheric gases on film chemistry. For very thick films, used in laser power or infrared optics, short crystal life may prevent completion of the coating. For high-speed roll coating systems, abrupt crystal failure can cause great amounts of ruined substrates.
Attempts have been made to reduce crystal failure and increase accuracy, e.g., through the use of sensors made of AT-cut quartz and through the use of water-cooled holders and/or sensor heads in order to maintain the temperature of the sensor between 20 degrees C. and 45 degrees C., in which temperature range the AT-cut quartz is “substantially temperature insensitive,” (see FIG. 1) in order to reduce thermally induced frequency shifts for low temperature processes.
That is, in the past, films have been deposited at elevated temperatures in order to attempt to alleviate stresses which result from the films being built up. However, because such elevated temperatures cause the sensor to move out of the “substantially temperature-insensitive region,” and result in frequency shifts in the thickness measurements of such sensor systems (see FIG. 1), prior systems have used cooling systems to try to counteract the effects of such heating, and to try to maintain the temperature in the substantially “temperature-insensitive” region.
For example, conventional quartz crystal based thin film thickness sensor systems utilize a water cooled stainless steel holder which uses a thin (0.010″ thick) quartz crystal disk to measure the thickness, in situ and real time, of a thin film deposition process. This technology, available since the early 1960's, is difficult to use when optical materials, such as magnesium fluoride, or silicon dioxide, are used in the coating process. These materials cause the crystal to act erratically and fail prematurely during the coating process, preventing the measurement and control function from taking place. It is thought that the intrinsic stresses that these materials have when deposited as thin films result in the quartz becoming strained microscopically. Typically, lenses to be coated are heated during coating to alleviate this stress.
The quartz sensor, placed near the structures (e.g., lenses) being deposited to monitor the process, has historically been water cooled at the same time, to minimize fluctuations in its reading due to temperature changes resulting from process heat (i.e., heat resulting from the process being used to deposit the coating). This cooling, unfortunately, compounds the stress problem on the crystal surface. Moreover, recent studies of standard sensor heads show that even with water-cooling, the crystal temperature can rise 20 to 30 degrees within a 10-minute process. For extended runs with high chamber temperatures, temperature increases can become considerably larger.
Others have attempted to generate temperature-frequency algorithms to try to cancel out the component of frequency change caused by temperature. Examples of such work include: (1) E. C. van Ballegooijen, “Simultaneous Measurement of Mass and Temperature using Quartz Crystal Microbalances” Chapter 5, Methods and Phenomena 7, C. Lu and A. W. Czandema, Editors, Applications of Piezoelectric Quartz Crystal Microbalances, Elsevier Publishing, New York, 1984, and (2) E. P. Eemisse, “Vacuum Applications of Quartz Resonators”, J. Vac. Sci. Technol., Vol. 12, No. 1, January/February 1975, pp 564-568.
A paradigm shift is underway in quartz crystal process monitoring. In many applications, crystals become the keys to success. No matter how significant a breakthrough may be in optics, be it materials, geometry, process design or application, if a thin film coating of any sophistication is required, the weak link is how accurately that film can be measured. As technology closes in on manipulating Angstrom-level properties of matter, the need for reliable thin film metrology rises to a new level of importance.
Film stress, adhesion failure, and extreme temperature effects have not been adequately dealt with. The current demands of nanotechnology, thin film displays, and high speed optical communications bring about an increased need for a quartz crystal monitor which reduces these inaccuracies and which reduces the frequency of such malfunctioning.